An apparatus is known from U.S. Pat. No. 2,559,227 of RIEBER, for generating high frequency shockwaves, which apparatus comprises a truncated ellipsoidal reflector 80 in which shockwaves are generated by discharge or electric arc between two electrodes converging to the first focal point of the ellipsoid, the object being to destroy a target situated in the second focal point of the ellipsoid, which is external to the truncated reflector 80 (see FIG. 3 and col. 7, line 51, to col. 9, line 30).
Electrodes 12 and 13 are produced in a highly conductive material such as copper or brass and are mounted on an insulator 26 which is supported in pivotal manner by means of a device 11a, 11b, so as to adjust the spacing between said electrodes (see col. 4, lines 42 to 53 and col. 8, lines 40 to 47).
With the RIEBER apparatus or any similar apparatus, the discharge or electric arc is produced between the electrodes due to the sudden discharge of a capacitor 11, by closing a high voltage switch (see FIG. 2B). According to the RIEBER apparatus, the circuit between the electrodes comprises a capacitor, with an associated self-inductance. It has been noted that the capacitor discharge is of damped oscillatory type. In other words, the capacitor is going to discharge and to re-charge in reverse at a lower voltage than the initial voltage which is very high, until depletion of the charges contained in the capacitor occurs.
Simultaneously, an electric arc and a plasma are established between the two electrodes of which the current will also be, by way of consequence, of damped oscillatory type, as can be understood with reference to FIGS. 1a, 1b and 1c of the present application which illustrates prior art. Accordingly, FIG. 1a illustrates the chronogram of voltages, while FIG. 1b illustrates the chronogram of currents established in the RIEBER type discharge circuit. It is found that when the circuit is closed at time t.sub.1, the voltage at the terminals of the electrodes rises suddenly to the value of the voltage at the terminals of the capacitors (see FIG. 1a). A low current is established between the two electrodes (FIG. 1b) due to the fact that, first the liquid in which the electrodes are immersed, and which is usually water, is still slightly electrically conductive, and second, that for reasons of safety and of arc ignition, a high resistance is provided in parallel to the capacitor supplying the electrodes.
After a certain time, namely after time t.sub.2, called latency time, the arc is established between the electrodes. At that moment, the current increases suddenly by several KA as is clearly illustrated in FIG. 1b. It is a known fact that the arc is constituted by a plasma whose resistance is extremely low (about 1/100 or 1/1000 Ohm) and it is the low value of this resistance which explains the importance of the oscillations of current (FIG. 1b) and of voltage (FIG. 1a) during the discharge of a capacitor in an RL type circuit.
The energy contained and dissipated by the arc contributes to the vaporization of the liquid in which the electrodes are immersed, and which is normally water, to the creation of a steam bubble and consequently to the formation of the shockwave. The quicker this energy is dissipated, the more efficient will be the shockwave.
It is thus found that, due to the oscillatory nature of the current, as illustrated in FIG. 1b, the supply of energy to the external medium is progressive, as clearly illustrated in FIG. 1c.
This explains how, the quicker the vaporization of the liquid is, the stronger the pressure wave will be and it will have a shorter rising time.
Thus, a great quantity of energy will have to be delivered to vaporize quantity of energy will have to be delivered to vaporize a sufficient quantity of liquid, and in particular water.
Yet, virtually all the currently known devices use discharges which are all of damped oscillatory type, as illustrated in FIGS. 1a and 1b, resulting in a progressive dissipation of the energy with time (FIG. 1c)
In commonly assigned EP-A-0 296 912 which is equivalent to U.S. Pat. No. 4,962,753, a first solution has been prepared for delivering suddenly or in a relatively short time, most of the energy stored by the charge of the capacitor of the discharge circuit between two electrodes. It was proposed to this effect, to increase the electric resistance on the path of the electric arc at least between the electrodes by interposition of a high resistance insulating element (32), between the arc-generating electrodes 12, 14. This solution is fully satisfactory when generating shockwaves whose initial pressure wave is substantially spherical.
However, said prior solution is difficult to implement mechanically because of the small dimensions of the electrodes and of the mechanical strength towards shockwaves. Moreover, the latency time problem is not solved in that the main aim of this particular solution is only to improve the discharge rate when this is established, which does not improve the reproducibility of the discharge, nor consequently the reproducibility and efficiency of the generated pressure waves, nor does it reduce the wear of the electrodes.
U.S. Pat. No. 3,559,435 of GERBER describes the use of a conductive liquid to provide a preferential conductive pathway for the current in order to form an arc where the current is established (see col. 5, line 4). The object is therefore to establish an arc and a growth of plasma between two electrodes in a conventional discharge. The aim of the recommended electrolyte is therefore to establish a preferential current between the electrodes in order to create a high conductive plasma (col. 1, line 55).
GERBER's solution does not in any way alter the configuration of the oscillating current which causes the wear of the electrode, or of a progressive supply of the energy to the external medium.